Nowadays, integrated circuits are typically formed on semiconductor substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive and insulative layers on the wafer. After deposition of each layer, etching is performed to create the circuitry functions. After a series of layers have been sequentially deposited and etched, the uppermost surface of the semiconductor substrate, i.e., the outer surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize or level off the semiconductor substrate surface.
So-called chemical mechanical polishing (CMP) is one of the accepted methods for this. This planarization method typically requires that the substrate, i.e., the semiconductor wafer, be mounted on a carrier or polishing head. The exposed surface of the substrate is then pressed against a rotating polishing pad. A controlled force is exerted on the substrate via the carrier head to press the substrate against the polishing pad. A polishing agent containing at least one chemically reactive substance and abrasive particles is supplied to the surface of the polishing pad.
A recurring problem in the CMP process is the so-called edge effect, i.e., the tendency to polish the edge of the substrate at a different rate than the center of the substrate. This typically results in over-polishing at the edge, i.e., the removal of too much material from the edge, particularly at the outermost 5 to 10 mm of a wafer of 200 mm in diameter.
Over-polishing reduces the overall flatness of the substrate and makes the edge of the substrate unsuitable for integrated circuit fabrication and therefore decreases the process yield.
To solve this problem, U.S. Pat. No. 6,251,215 discloses a retaining ring be made of two portions, a first portion being made of a rigid material, namely a metal portion, and a second portion of a plastic material, which is less rigid, so that, on the one hand, it can be subjected to abrasion, and, on the other hand, it will not damage the semiconductor wafer when contacting it.
Owing to the edge conditions that prevail in chemical mechanical polishing, U.S. Pat. No. 6,251,215 discloses that the plastic portion of the retaining ring and the metal ring are bonded to one another with an epoxy adhesive. Alternatively, it is disclosed that the two portions are joined together with a press fit.
In practice, both solutions prove to be inadequate.
While the plastic portion is held securely on the metal portion when the two portions are bonded with epoxy adhesive, the reconditioning of the retaining ring after the plastic portion has been subjected to a certain amount of abrasion presents problems. The current practice is to send the complete retaining rings to the manufacturer where the plastic portion is mechanically removed and the metal portion is then heated up to approximately 200° C. to thermally decompose the adhesive residues thereon. Subsequently, the metal portion has to be sandblasted in order to remove final residues of the adhesive, and only then can a new plastic ring be adhesively attached thereto.
Owing to this time-consuming and costly procedure, the retaining rings as such become very expensive. In addition, the metal portions, which are more expensive to produce than the plastic portions, only withstand a small number of cycles of reconditioning, in particular, on account of the temperature treatment for thermal decomposition of the adhesive and the sandblasting treatment that is subsequently required.
Exchanging a used plastic ring when metal and plastic portions are joined with a press fit is easier, but a press fit for joining the plastic and metal portions has proven unsuitable for reliably withstanding the forces that occur during the polishing process.
The present invention, relating to a retaining ring that can be manufactured more cost-effectively and, in particular, fitted more cost-effectively with a new plastic part, provides for ameliorating at least some of the disadvantages of the prior art. These and other advantages of the present invention will be apparent from the description as set forth below.